What causes 60-cycle hum sometimes when I try to hook up an on-stage guitar amp? Why doesn’t the Red-Eye normally need a Ground Lift Switch? How do you solve the problem?
You only run into problems with AC ground loops and 60-cycle hum when AC grounds from two different AC power circuits are connected together. Specifically, it the ground of AC power running the stage is connected to the ground of AC power running a venue’s sound system are connected together through any DI.
The Red-Eye is normally used to connect an isolated acoustic instrument to a venue’s mix board so there is no connection to any on-stage AC ground. In the normal situation, stage ground is not connected to the Red-Eye, so there is no problem.
With the Red-Eye connected to the venue’s sound system ground via an XLR cable, connecting a pedal board to the Red-Eye's Effects Loop is not normally a problem because pedal (wall-wart) power is isolated and there is no connection to stage AC power ground.
An example that could cause 60 Hz hum would be connecting a Red-Eye to a venue's mix board ground via the XLR connection and also to an on-stage guitar amp ground via a ¼-inch cable. The AC ground is different for each AC circuit and tying them together through a DI like the Red-Eye would induce 60 Hz hum in the grounded device connected with a single-ended ¼-inch cable, i.e. the guitar amp.
If you need to connect to an on-stage amp or some other AC-grounded device via the Red-Eye's Effects Out 1/4-inch connector and at the same time connect to a venue's remote mix board you will likely encounter 60-cycle hum. Some guitar amps have a ground-lift switch which solves the problem. If there is no ground lift on the guitar amp, you can use a ground-lift adapter to connect between the Red-Eye's XLR output and the cable to the mix board.
Here's a link to a popular XLR ground-lift adapter...
Others, as inexpensive as $10, are available through Amazon.
If you do use an XLR ground lift adapter, it will interrupt Phantom Power from the mix board and the Red-Eye will need to run on its internal 9-volt battery.
FREQUENTLY ASKED QUESTIONS
How is the gain managed as the signal moves through the Red-Eye Preamp?
A fixed amount of gain is applied by the single-channel Red-Eye Preamp's high input impedance field-effect input amplifier. The Red-Eye Twin has gain controls for both channels and when the controls are set to 90 percent of full gain, the signal levels are the same as the single-channel Red-Eye.
The following levels are output relative to an Instrument Input's reference level of 0 db. (Output levels are specified with the Boost Button off).
• From the Instrument Input to the Effects Output the gain is -1 db. (In other words the output voltage level is 80% of the input)
• From the Instrument Input to the Balanced XLR Output, the gain is -5 db. (In other words the output voltage level is 33% of the input)
• From the Effects Input to the Balanced XLR Output, the gain is -5 db. (In other words the output voltage level is 33% of the input)
The Boost Button affects both the Effects Output level and the XLR Output Level. With the Boost Volume control set to minimum, the Boost Button adds +3 db to the output levels. (This is the threshold of clearly hearing a boost in the signal) With the Boost Volume control set to maximum, the Boost Button adds +8 db to the Output Levels.
Why is there no input level control on the single-channel Red-Eye?
I was able to simplify the Red-Eye Preamp and eliminate an input level control by providing lots of headroom. The Preamp is specified to handle 1 Volt peak-peak Instrument Input signal levels without distortion. That's about 3 times what most passive pickups can generate. The Red-Eye can actually handle about 2 Volts peak-peak and has no time constant associated with recovery after being overdriven. In other words, it does not "block", "gasp", or "quack" if overdriven...only the offending signal peak will be neatly clipped and will cause no longer-term effects.
The Red-Eye Twin does have gain controls so that two instruments can be balanced.
How did you determine the output levels?
For the Effects Output Level, I did some testing of a few popular effects boxes and pedals and discovered a number of them have little headroom. To make sure the Red-Eye Preamp did not make matters worse and drive the Effects into overload, I reduced the Effects Output level by 1 db below what the Instrument generates. Not much, I admit, but it makes sure when the Red-Eye Preamp's Effects Loop is used the effects may sound cleaner than without the Red-Eye Preamp.
For the Balanced XLR Output Level, the differential output voltage level to the XLR Output is determined by the turns-ratio of the transformer I use. Essentially, the transformer has three turns in the primary winding for each turn in the output winding, which makes the output level 1/3 of the input level. That translates to 5 db below the Instrument Input level. This output level is equivalent to a pretty "hot" microphone. I'm sure there may be some simple mix boards that may not be able to handle this fairly high level, but I designed this unit to have the highest signal-to-noise ratio possible, and that means a hot drive level. Any good mix board will have an input level control for each channel, so all inputs can be balanced and controlled to optimize signal levels through the board. I'm counting on the sound technician to understand how to do this!
How does the Red-Eye Preamp help piezoelectric pickups sound more like the instrument?
I wish I could claim magical insight on how to make musical instruments sound more acoustic, but there's really nothing tricky in the Red-Eye Preamp. Of course, piezoelectric pickups require the very high input impedance offered by the Red-Eye Preamp's field-effect transistor input circuit. This prevents "loading" of the pickup and keeps the frequency response of the pickup more nearly flat. But basically, the preamp is just simply a very HiFi circuit. Flat response beyond the audio band, extremely low distortion, lots of headroom, and very low noise. There is no "trick" EQ or compression or anything else messing with the signal from the instrument...I simply try to preserve the signal from the pickup precisely and exactly through the entire signal path.
The lower frequencies are so clear when using the XLR output to the mix board. What makes this difference?
If there's a secret, it's the balanced output transformer. I must have bought over 20 different transformers to test. Some were quite expensive (over $50) and others were less than $2. I was testing for flat frequency response, low, smooth phase shift and low intermodulation distortion. I did not get to test the well engineered Jensen transformers because they were all too big to fit into the Red-Eye Preamp case.
One small transformer stood out way above the rest. Its low-frequency response extended smoothly down to below 20 Hz and the high-frequency response went cleanly above 40 KHz. This transformer allowed me to couple the high performance of the electronic preamplifier to the XLR output without losing any fidelity. Turns out, this transformer was not even designed for audio use...it was normally used in computer modems.
How come the Red-Eye Preamp has a Treble Control, but no other EQ?
Part of the answer is that the Red-Eye has flat response to 18 KHz, and you can hear the scrachiness of a fiddle bow. Many other DI boxes don't reproduce those high frequencies at all. Musicians, especially fiddle and mando players, like to adjust the treble brilliance to suit their particular instrument's pickup. The bass response of the Red-Eye is flat down to 20 Hz and most musicians like the way that sounds.
The Red-Eye preamps are designed for performance in situations where a good sound system is being used. Such systems always have several bands of EQ that can be applied to each channel separately. Often when a musician with EQ controls on-stage tweeks his EQ, the sound technician at the mixboard tweeks his EQ controls to compensate. When a competent sound system is used by a good sound technician, I believe it's best for the on-stage preamp to supply the cleanest high-fidelity sound possible and let the sound system provide any EQ necessary for the room.
How is the Red-Eye Preamp resistant to neon sign buzz and cell phone noise?
The Red-Eye has a filter circuit that cuts the radio frequency ranges generated by that kind of noise. The Red-Eye has lots of headroom too, so that any such noise does not overload the Red-Eye's input circuits and generate intermodulation distortion noise that might propagate through additional stages of Red-Eye noise filtering.
What does the Power Check Light do?
The Power Check light on the Red-Eye Preamp and on the Red-Eye Twin flashes for one second when the Red-Eye is powered up if the power is good. If you plug in XLR Phantom Power first, it will measure the voltage and flash for one second if the Phantom power is good. If you plug in an instrument first, the Red-Eye will power up on the battery and flash for one second if the battery is in the first 3/4 of it's life. It will not flash if the battery is below 25% of capacity. You can probably play another half-dozen gigs on the battery after the light quits flashing.
The Red-Eyes are designed to normally run on Phantom Power and will automatically use it instead of the battery whenever possible. The battery is there for insurance in case you run into an old mixboard that doesn't supply Phantom Power. Battery life is about 200 hours for the Red-Eye Preamp and about 100 hours for the Red-Eye Twin. Some players I know have not had to change the battery in over two years because they normally get Phantom Power.